Monday, January 30, 2006

Science Series: Sunlight, Moonlight

Gravity and light. Silver moonlight, just a reflection of the sun's light. What is sunlight?

Sunlight in the broad sense is the total spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered by the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. This is usually during the hours known as day. Near the poles in summer, sunlight also occurs during the hours known as night and in the winter at the poles sunlight may not occur at any time. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and heat. Radiant heat directly produced by the radiation of the sun is different from the increase in atmospheric temperature due to the radiative heating of the atmosphere by the sun's radiation.

the heliospheric magnetic field of the sun: this field extends from the Sun's equatorial plane throughout the entire Solar System, and can be considered its largest structure. The shape of the current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind)

Okay. And moonlight is a pale reflection of sunlight.

Moonlight is the light that is perceived as coming from the moon. Moonlight is in fact sunlight reflected from the Moon. The intensity of moonlight varies greatly depending on the current lunar phase; but even the full moon provides only a faint illumination of about 0.2 lx (so full moon is about 500,000 times fainter than the sun). One cannot read books or discern colors under moonlight.

Why is moonlight so much more poetic than sunlight? Where did the moon come from? This is the current thinking:

Recently, the giant impact hypothesis has been considered a more viable scientific theory for the moon's origin than the coformation or condensation theory. The Giant Impact theory holds that the Moon formed from the ejecta resulting from a collision between a very early, semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia. The material ejected from this impact would have gathered in orbit around earth and formed the moon.

At the center of the Sun, where its density reaches up to 150,000 kg/m3 (150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the energy that keeps the Sun in a state of equilibrium. About 8.9×1037 protons (hydrogen nuclei) are converted to helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15×1016 tons of TNT per second).

The core extends from the center of the Sun to about 0.2 solar radii, and is the only part of the Sun where an appreciable amount of heat is produced by fusion: the rest of the star is heated by energy that is transferred outward. All of the energy of the interior fusion must travel through the successive layers to the solar photosphere, before it escapes to space.

The high-energy photons (gamma and X rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as constant absorption and re-emission at lower energies in the solar mantle (see below). Estimates of the "photon travel time" range from as much as 50 million years (Richard S. Lewis, The Illustrated Encyclopedia of the Universe, Harmony Books, New York, 1983, p. 65) to as little as 17,000 years. Upon reaching the surface after a final trip through the convective outer layer, the photons escape as visible light. Neutrinos are also released in the fusion reactions in the core, but unlike photons they very rarely interact with matter, and so almost all are able to escape the Sun immediately.

My college roommate told me recently that he met a person once who legally changed his last name to "Moonfire". Just watch that you don't get a Moonburn skinny dipping at night. That ain't no reflection!